The individual secrecy capacity of the Gaussian SISO and degraded Gaussian MIMO multi-receiver wiretap channel

We study secure communication in which a transmitter wants to send confidential messages to an arbitrary number of receivers in the presence of an external eavesdropper. We consider two classes of Gaussian channels: The Gaussian single-input single-output (SISO) multi-receiver wiretap channel and the degraded Gaussian multiple-input multiple-output (MIMO) multi-receiver wiretap channel. These two channels belong to the class of degraded multi-receiver wiretap channels. In previous literature the secrecy capacity regions of these two channels were established under the conservative joint secrecy constraint. Thus, we focus our work on the less conservative individual secrecy constraint, which is characterized by a higher throughput. We establish the individual secrecy capacity region for these two cases. The achievability follows from the individual secrecy capacity established for the discrete memoryless degraded wiretap channels where coding is performed using Gaussian signals. On the other hand, the converse is established by adapting the techniques used for formulating the joint secrecy capacity region to the individual secrecy constraint.